The surveillance of ground topography is well known in the art. It is frequently the case that an aircraft or a satellite includes an image capture device such as a charge coupled device (CCD). In ground surveillance it is highly desirable to detect whether there has been a material failure in a man-made object such as a road, a pipeline, an electrical grid, or other man-made structures of practical interest. When detected, a determination is made if remedial action must be taken. Often times a visual inspection of ground topography is provided by a land-based crew that traverses an area by vehicle or foot, to determine if there is a material failure. Airborne photographic systems can also be used for capturing images of adjacent areas. These images are then reviewed to determine if there is a material failure.
In detecting failures in gas pipelines there is a particular problem as the pipeline is typically buried beneath ground level. In such cases it is difficult to make direct visual assessment of the failures in pipelines. The fact remains that when failures occur and they are manifest by the leakage of the pipeline contents, the leaked material produces a characteristic trace or signal. Typically, failures in pipelines are currently determined by having personnel walk the pipeline on a periodic and costly basis with means to detect the trace or signal.
It is understood that pipelines usually carry petroleum, natural gas, refined petroleum or gas products, chemicals, mineral ore slurries and other fluid or fluidized substances or mixtures.
When electromagnetic radiation from either a natural or man-made source interacts with matter a number of phenomena may occur including scattering, absorption, transmission and reflection. When the interactions of electromagnetic radiation and matter are carefully examined, analyzed, and represented in an ordered fashion as a function of wavelength, frequency, or time this is referred to as a spectral or spectroscopic analysis. During spectroscopic analyses different materials exhibit different scattering, absorption, reflection and transmission characteristics. These characteristics are determined by the chemical and physical structure of the materials. When a set of these characteristics are determined to a given level of certainty, as with the use of known test subjects, these spectroscopic results may be referred to as reference spectral signatures or reference spectra. Natural gas characteristically contains a mixture of methane, ethane, and small amounts of other gases. Gas generated by the decomposition of organic matter henceforth referred to as swamp gas, only contains methane. It is highly desirable for a method of detection to be able to distinguish between gases released as a result of a failure in a pipeline or holding container versus swamp gases in this manner avoiding false alarms. It is possible to use methods involving illuminants and their corresponding interaction with the probed areas to detect the presence of various chemical compositions and mixtures as described by Windig in U.S. Pat. No. 5,481,476. This patent describes the chemometric analysis of data. This patent provides for a quantitative method of determining remotely the nature of chemicals detected by the probe. In many cases this provides the required certainty for avoiding false alarms and potentially the capability of identifying the source(s) of the detected species. This same methodology can be applied to species other than natural gas
Electromagnetic radiation can be directed onto a test subject by any of a variety of means. Commonly lasers are used but other means such as the use of antennas for radio and microwave electromagnetic energy may be used. Hereafter when electromagnetic radiation is directed onto a test subject it is referred to as an illuminant.
Raman spectral signatures for natural gas components are well known. Hansen et. al., Appl. Spectrosc. 55(1), p. 55 (2001) have recently reported laboratory studies of natural gas samples at high pressure.